This application claims the benefit of International Application Number PCT/SE99/01336, which was published in English on Feb. 17, 2000.
The present invention relates to a method and a device for point cooling, particularly for point cooling of an accessible surface or of a detail with a sharp edge and/or a varying material thickness at injection moulding or casting.
Injection moulding is a manufacturing process, wherein plastic is injected into a moulding tool under pressure and solidifies. The moulding tool is substantially a negative of the detail which is to be manufactured. Typical injection pressures vary from 5000 to 20000 psi. Since these high pressures exist, the moulding tool, which is often composed of a two part mould, has to be held clamped in a closed position during injection and cooling. This clamping force shall be considerably high.
Injection moulding may be used for manufacturing a large number of details with a very high precision. Tolerances better than 0.0025 mm are easily achieved with a suitable combination of moulding tool design, material and detail design. Furthermore, injection moulding is a high capacity process. Cycle times vary from a few seconds to several minutes depending on the size and form of the injection moulded detail.
A problem with the technique, particularly in injection moulding of details with sharp edges, varying material thickness or narrow cavities, is the occurrence of so-called hotspots. These deformations arise when part of the detail is solidifying at different speeds in different, often substantially reverse directions.
A traditional way of solving this problem is to form cooling channels to the hotspot-exposed part and into these channels insert cooling rods, i.e. rods with high thermal conductivity which are cooled. Alternatively, cooling water or a cooled concentrated gas is directed into the cooling channels. Hereby, also the cycle time may be very considerably shortened. Sometimes this cooling may, however, be unsufficient.
The German patent document DE 3 322 312 describes a cooling method in manufacturing an injection moulded detail in a moulding tool with a core. In the core there is a recess, in which a conduit for liquid gas is existing. This conduit is, via an access conduit provided with a magnetic valve, in connection with a tank containing a liquid gas, e.g. nitrogen, air, carbon dioxide or argon. Furthermore, the recess has an exhaust conduit, wherein there is a thermometer. A regulation and control unit may be used to control the magnetic valve by means of temperature measured in the reverse conduit. Cooling will be existing over a relatively large surface.
A further method using liquid carbon dioxide as cooling medium is dry precision cooling, DPC, which method is used for cooling during dry precision grinding. The method comprises that a mixture of carbon dioxide snow and gas are supplied to the tool and the workpiece in a thin, highly effective cooling jet.
However, it is desirable to find a method and a device, which effectively may cool very small surfaces of an arbitrary kind. Surfaces of particular interest in this respect are accessible open surfaces, e.g. at heat generating electronic components or at human tissue, and surfaces at details with a shart edge and/or varying material thickness at injection moulding or casting.
Thus, it is an object of the present invention to provide a method and a device for effective, fast and reliable point cooling.
A further object of the invention is to provide a method and a device, particularly for point cooling of an accessible open surface.
Another object of the invention is to provide a method and a device, particularly for point cooling of a detail with a sharp edge and/or varying material thickness at injection moulding or casting.
This and other objects are attained according to one aspect of the invention by a method for point cooling, which method comprises providing a cooling channel with a defined end surface and an outlet valve separated therefrom, a container with liquid carbon dioxide and an access conduit, provided with a nozzle, connected to said container and localized to said cooling channel. Liquid carbon dioxide is brought to flow from the container and out through said nozzle.
In this respect the overpressure of the container, the throttling of the outlet valve and the design/size of the nozzle are to be chosen so that the pressure in the channel is held over the so-called triple point pressure of carbon dioxide, i.e. 5.18 bars. Furthermore, the distance between the nozzle and the end surface shall be adapted so that mainly all carbon dioxide is gasified at the end surface. A corresponding inventive device for point cooling comprises said parts arranged in a described way.
Preferably, the pressure in the channel is held within the interval 5.18-8 bars. The most effective cooling is achieved, if the pressure in the channel is held substantially immediately above the triple point pressure.
A corresponding inventive method and device, particularly intended for point cooling of an accessible surface, particularly comprise that the cooling channel with its end surface is composed of the interior of a tubular means with an end surface of a material of a high thermal conductivity.
An inventive method and device, particularly intended for point cooling of an injection moulded or cast detail with a sharp edge or a varying material thickness particularly comprise that the cooling channel is formed in a moulding tool intended for manufacturing the detail, said end surface being localized in the vicinity of the sharp edge or the varying material thickness of the detail.
An advantage of the point cooling method and device according to the present invention is that cooling is applied very rapidly and effectively and of very small surfaces.
A further advantage of the invention is that a smooth and continuous cooling effect is achieved.
Yet a further advantage of the invention is that the cycle time for injection moulding or casting may be shortened.
Laboratory experiments have i.a. shown that it is extremely important to hold the pressure above the triple point pressure both momentarily and in long term to avoid any instabilities, such as a heavily varying pressure caused thereby that some carbon dioxide snow does not evaporate but at least partly plugs up the outlet. On the other hand, too high pressures may also result in similar instabilities. Here, all liquid is not evaporated at the end surface but some amount continues to the outlet, where it will condense to snow.
More advantages of the invention will be apparent in the following description.